1. Field of the Invention
This invention relates to thin-walled container lids, and more particularly to stackable, hot-molded lids of foil-thin thermoplastic substance.
2. Description of the Related Art Including Information Disclosed Under 37 CFR .sctn..sctn.1.97-1.99
Lids of the above type as heretofore produced usually were provided with circular peripheral sealing ribs of somewhat U-shaped cross section adapted to be introduced into container openings, said ribs having (1) inner circular leg portions or walls and (2) having outer circular leg portions or walls, the latter transitioning into circular outer flanges while the inner leg portions or walls transition into the actual lid body or closure wall. The two leg portions of a sealing rib also form an annular groove which is open at the top of the lid and which tapers to a smaller opening towards the top. Molded into the annular bottom wall of a sealing rib is an annular stacking ring means or configuration which projects slightly downwardly and is adapted to fit into the circular opening or slot formed by the annular groove of an underlying identical lid, such stacking ring means having annular concentric upright stacking seating surfaces disposed on both of its sides.
Stackable, thin-walled container lids of this kind as revealed in German Publication DE-GM No. 78 06 380 each have a ring-shaped, flat outwardly radially-extending peripheral flange which surrounds the sealing rib and serves as a retainer rim to be fused or sealed to the opening of the container. The stacking seating surfaces of these known lids are rounded at the bottom of the sealing rib and interact with correspondingly rounded edges of an underlying identical lid at those transition points of the latter which are disposed between the inner peripheral wall of the sealing rib and the lid bottom or body, and/or at the transition points which are disposed between the outer peripheral wall of the underlying sealing rib and the associated peripheral flange or retainer rim. This is for the purpose of enabling the lowermost lid of a stack of lids to be readily forcibly laterally pushed or fed out of the remainder of the stack.
However, these known configurations cannot operate with lids of the snap type, i.e. those container lids which have, at their outer peripheries, locking rim portions that are folded down axially, gripping over the opening rims of the containers.
In addition, the prior configurations provided in the stackable, thin-walled container lids revealed in German Publication DE-GM No. 78 06 380 have the drawback that the axial load bearing capacity of the stacked, cooperable seating surfaces is relatively small due to the rounding of the transition points at such transition surfaces and the rounded transitions of the peripheral sealing rib walls to the actual lid bodies and peripheral flanges. Therefore, container lids of this known stacking type tend to jam at the mutually stacked sealing ribs when an excessive working axial pressure is exerted on a stack of such lids. This danger of mutual jamming of stacked container lids makes it virtually impossible to safely use such known lids in automatic container filling and sealing machines. For, if two stacked container lids were to jam, the feeding of the lids to the sealing station would be unquestionably interrupted. Such breakdown of the operation can then only be eliminated by costly means, and with considerable loss of time.